1. Field of the Invention
The present invention relates to a continuous process for the preparation of propylene oxide.
2. Description of the Prior Art
It is known that in processes for preparing propylene oxide by the oxidation of propylene with a molecular oxygen-containing gas in a liquid phase, a number of by-products are formed, some of which are particularly troublesome due to the reactions which they provoke or difficult separation they involve.
It is thus known that formic acid, and to a lesser extent acetic acid, formed during the reaction, react quickly with epoxides (H. W. Gibson -- Chem. rev. 1960 -- 5 -- 675; W. J. HIckinbottom et al. J. Chem. Soc. -- 1954 -- 4200) and particularly with propylene oxide, especially in the presence of small quantities of water, to give not only propylene glycol and its esters, but also heavier products consisting mainly of polypropylene glycols more or less esterified by the acids.
On the other hand, alcohols are equally produced, during the oxidation reaction of propylene, which give esters by reaction with the acids. Among these esters, methyl formate is particularly troublesome for its boiling point is very near that of propylene oxide, which makes separation thereof difficult and expensive. The other esters, such as isopropyl or propylene glycol formate, can also create complications during separation operations due to their modiocre stability, especially in the presence of water.
It has been noted that the production of heavy products was all the more significant since the concentration in acids, particularly in formic acid, is higher in the reaction mixture. The same holds true for the esters.
Under normal conditions of propylene oxidation in liquid phase using molecular oxygen, acids and propylene oxide are found in the reaction mixture in variable quantities, but the ratio of concentrations thereof is practically constant when the reaction is carried out in a given solvent. This practically constant ratio is the consequence of the mechanism of formation of propylene oxide by the oxidation of propylene in liquid phase (A. V. Bobolev et coll. -- Zh. Fiz. Khim -- 1970 -- 44 -- 4 -- 1028).
It has been proposed (French Pat. No. 1,414,707) in order to remove this formic acid, to carry out the oxidation of propylene in liquid phase in the presence of a solid neutralizing agent such as sodium carbonate suspended in the liquid. Such a process however, involves the formation of CO.sub.2 and necessitates a filtration operation of the produced sodium formate which is insoluble in the reaction mixture, and therefore the process is rendered difficult.
It has also been proposed (U.S. Pat. No. 2,784,202) to treat the liquid phase leaving he vessel with a solution of sodium bicarbonate. However, such a process presents the drawback of making the process for recovery of the acid more complicated.
There is a need therefore, for a solution to the above problems and particularly, there is a need for a technique of removing or reducing the acid formed as a by-product in the reaction solution.